This project is concerned with young children's facility with, and preference for, to conceptual organizational principles (taxonomic class and complementary relationship) in a memory situation. Factors influencing their facility with these two grouping principles are being systematically investigated as is the developmental preference for the use of one or the other over a wide range. Generally, on some measures young children seem equally facile with both taxonomic and complementary principles. Other measures show a decided superiority for complementary relations. Preference for complementary grouping persists for a long time and on some verbal measures is intrusive.